From the first link, "The Microsoft Enterprise Library is a collection of reusable software components (application blocks) designed to assist software developers with common enterprise development cross-cutting concerns (such as logging, validation,
data access, exception handling, and many others). Application blocks are a type of guidance; they are provided as source code, test cases, and documentation that can be used "as is," extended, or modified by developers to use on complex, enterprise-level
line-of-business development projects."